Designed fluorescent protein cages as fiducial markers for targeted cell imaging.

Understanding how proteins function within their cellular environments is essential for cellular biology and biomedical research. However, current imaging techniques exhibit limitations, particularly in the study of small complexes and individual proteins within cells. Previously, protein cages have been employed as imaging scaffolds to study purified small proteins using cryo-electron microscopy (cryo-EM). Here we demonstrate an approach to deliver designed protein cages - endowed with fluorescence and targeted binding properties - into cells, thereby serving as fiducial markers for cellular imaging. We used protein cages with anti-GFP DARPin domains to target a mitochondrial protein (MFN1) expressed in mammalian cells, which was genetically fused to GFP. We demonstrate that the protein cages can penetrate cells, are directed to specific subcellular locations, and are detectable with confocal microscopy. This innovation represents a milestone in developing tools for in-depth cellular exploration, especially in conjunction with methods such as cryo-correlative light and electron microscopy (cryo-CLEM).

Pan-cancer landscape of epigenetic factor expression predicts tumor outcome.

Oncogenic pathways that drive cancer progression reflect both genetic changes and epigenetic regulation. Here we stratified primary tumors from each of 24 TCGA adult cancer types based on the gene expression patterns of epigenetic factors (epifactors). The tumors for five cancer types (ACC, KIRC, LGG, LIHC, and LUAD) separated into two robust clusters that were better than grade or epithelial-to-mesenchymal transition in predicting clinical outcomes. The majority of epifactors that drove the clustering were also individually prognostic. A pan-cancer machine learning model deploying epifactor expression data for these five cancer types successfully separated the patients into poor and better outcome groups. Single-cell analysis of adult and pediatric tumors revealed that expression patterns associated with poor or worse outcomes were present in individual cells within tumors. Our study provides an epigenetic map of cancer types and lays a foundation for discovering pan-cancer targetable epifactors.

Screen time: an unbiased search for histone mutations that affect quiescence and chronological aging.

Quiescence, reversible cell cycle arrest, is essential for survival during nutrient limitations and the execution of precise developmental patterns. In yeast, entry into quiescence is associated with a loss of histone acetylation as the chromatin becomes tightly condensed. In this issue, Small and Osley performed an unbiased screen of mutations in histone H3 and H4 amino acids in budding yeast and identified histone residues that are critical for quiescence and chronological lifespan. The results indicate that multiple histone amino acids, likely affecting nucleosome structure and a wide range of chromatin-associated processes, can promote or inhibit quiescence entry. Many of the same histone amino acids are also critical regulators of chronological lifespan.

Soft strain-insensitive bioelectronics featuring brittle materials.

Advancing electronics to interact with tissue necessitates meeting material constraints in electrochemical, electrical, and mechanical domains simultaneously. Clinical bioelectrodes with established electrochemical functionalities are rigid and mechanically mismatched with tissue. Whereas conductive materials with tissue-like softness and stretchability are demonstrated, when applied to electrochemically probe tissue, their performance is distorted by strain and corrosion. We devise a layered architectural composite design that couples strain-induced cracked films with a strain-isolated out-of-plane conductive pathway and in-plane nanowire networks to eliminate strain effects on device electrochemical performance. Accordingly, we developed a library of stretchable, highly conductive, and strain-insensitive bioelectrodes featuring clinically established brittle interfacial materials (iridium-oxide, gold, platinum, and carbon). We paired these bioelectrodes with different electrochemical probing methods (amperometry, voltammetry, and potentiometry) and demonstrated strain-insensitive sensing of multiple biomarkers and in vivo neuromodulation.

Wearable microneedle-based electrochemical aptamer biosensing for precision dosing of drugs with narrow therapeutic windows.

Therapeutic drug monitoring is essential for dosing pharmaceuticals with narrow therapeutic windows. Nevertheless, standard methods are imprecise and involve invasive/resource-intensive procedures with long turnaround times. Overcoming these limitations, we present a microneedle-based electrochemical aptamer biosensing patch (µNEAB-patch) that minimally invasively probes the interstitial fluid (ISF) and renders correlated, continuous, and real-time measurements of the circulating drugs' pharmacokinetics. The µNEAB-patch is created following an introduced low-cost fabrication scheme, which transforms a shortened clinical-grade needle into a high-quality gold nanoparticle-based substrate for robust aptamer immobilization and efficient electrochemical signal retrieval. This enables the reliable in vivo detection of a wide library of ISF analytes-especially those with nonexistent natural recognition elements. Accordingly, we developed µNEABs targeting various drugs, including antibiotics with narrow therapeutic windows (tobramycin and vancomycin). Through in vivo animal studies, we demonstrated the strong correlation between the ISF/circulating drug levels and the device's potential clinical use for timely prediction of total drug exposure.

Bruins-in-Genomics: Evaluation of the impact of a UCLA undergraduate summer program in computational biology on participating students.

Recruiting, training and retaining scientists in computational biology is necessary to develop a workforce that can lead the quantitative biology revolution. Yet, African-American/Black, Hispanic/Latinx, Native Americans, and women are severely underrepresented in computational biosciences. We established the UCLA Bruins-in-Genomics Summer Research Program to provide training and research experiences in quantitative biology and bioinformatics to undergraduate students with an emphasis on students from backgrounds underrepresented in computational biology. Program assessment was based on number of applicants, alumni surveys and comparison of post-graduate educational choices for participants and a control group of students who were accepted but declined to participate. We hypothesized that participation in the Bruins-in-Genomics program would increase the likelihood that students would pursue post-graduate education in a related field. Our surveys revealed that 75% of Bruins-in-Genomics Summer participants were enrolled in graduate school. Logistic regression analysis revealed that women who participated in the program were significantly more likely to pursue a Ph.D. than a matched control group (group x woman interaction term of p = 0.005). The Bruins-in-Genomics Summer program represents an example of how a combined didactic-research program structure can make computational biology accessible to a wide range of undergraduates and increase participation in quantitative biosciences.

Prereplication complex proteins get caught moonlighting.

In this issue of PLOS Biology, Lattmann and colleagues report a new function for proteins of the DNA prereplication complex promoting the anchor cell to invade through the basement membrane and initiate vulval development in Caenorhabditis elegans.

Stressed-out yeast do not pass GO.

Using microfluidics and imaging, Argüello-Miranda et al. (2021. J. Cell Biol.https://doi.org/10.1083/jcb.202103171) monitor the response of individual yeast cells to nutrient withdrawal. They discover that cells arrest not only in the early G1 phase as expected, but also later in the cell cycle, and that an endoplasmic reticulum stress-induced transcription factor, Xbp1, is critical for arrest at other cell cycle phases.

20 years of Developmental Cell: Looking forward.

In our 20th anniversary year, we reflect on how fields have changed since our first issue and here look to the future. In this collection of Voices, our writers speculate on the future: in terms of philosophy, cell states, cell processes, and then how to model cell systems.

Is There a Histone Code for Cellular Quiescence?

Many of the cells in our bodies are quiescent, that is, temporarily not dividing. Under certain physiological conditions such as during tissue repair and maintenance, quiescent cells receive the appropriate stimulus and are induced to enter the cell cycle. The ability of cells to successfully transition into and out of a quiescent state is crucial for many biological processes including wound healing, stem cell maintenance, and immunological responses. Across species and tissues, transcriptional, epigenetic, and chromosomal changes associated with the transition between proliferation and quiescence have been analyzed, and some consistent changes associated with quiescence have been identified. Histone modifications have been shown to play a role in chromatin packing and accessibility, nucleosome mobility, gene expression, and chromosome arrangement. In this review, we critically evaluate the role of different histone marks in these processes during quiescence entry and exit. We consider different model systems for quiescence, each of the most frequently monitored candidate histone marks, and the role of their writers, erasers and readers. We highlight data that support these marks contributing to the changes observed with quiescence. We specifically ask whether there is a quiescence histone "code," a mechanism whereby the language encoded by specific combinations of histone marks is read and relayed downstream to modulate cell state and function. We conclude by highlighting emerging technologies that can be applied to gain greater insight into the role of a histone code for quiescence.

Methylation of histone 4's lysine 20: a critical analysis of the state of the field.

Chromatin is a highly dynamic structure whose plasticity is achieved through multiple processes including the posttranslational modification of histone tails. Histone modifications function through the recruitment of nonhistone proteins to chromatin and thus have the potential to influence many fundamental biological processes. Here, we focus on the function and regulation of lysine 20 of histone H4 (H4K20) methylation in multiple biological processes including DNA repair, cell cycle regulation, and DNA replication. The purpose of this review is to highlight recent studies that elucidate the functions associated with each of the methylation states of H4K20, their modifying enzymes, and their protein readers. Based on our current knowledge of H4K20 methylation, we critically analyze the data supporting these functions and outline questions for future research.

Intron retention is a robust marker of intertumoral heterogeneity in pancreatic ductal adenocarcinoma.

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer with a 5-year survival rate of <8%. Unsupervised clustering of 76 PDAC patients based on intron retention (IR) events resulted in two clusters of tumors (IR-1 and IR-2). While gene expression-based clusters are not predictive of patient outcome in this cohort, the clusters we developed based on intron retention were associated with differences in progression-free interval. IR levels are lower and clinical outcome is worse in IR-1 compared with IR-2. Oncogenes were significantly enriched in the set of 262 differentially retained introns between the two IR clusters. Higher IR levels in IR-2 correlate with higher gene expression, consistent with detention of intron-containing transcripts in the nucleus in IR-2. Out of 258 genes encoding RNA-binding proteins (RBP) that were differentially expressed between IR-1 and IR-2, the motifs for seven RBPs were significantly enriched in the 262-intron set, and the expression of 25 RBPs were highly correlated with retention levels of 139 introns. Network analysis suggested that retention of introns in IR-2 could result from disruption of an RBP protein-protein interaction network previously linked to efficient intron removal. Finally, IR-based clusters developed for the majority of the 20 cancer types surveyed had two clusters with asymmetrical distributions of IR events like PDAC, with one cluster containing mostly intron loss events. Taken together, our findings suggest IR may be an important biomarker for subclassifying tumors.

Splicing Busts a Move: Isoform Switching Regulates Migration.

Cell migration is essential for normal development, neural patterning, pathogen eradication, and cancer metastasis. Pre-mRNA processing events such as alternative splicing and alternative polyadenylation result in greater transcript and protein diversity as well as function and activity. A critical role for alternative pre-mRNA processing in cell migration has emerged in axon outgrowth during neuronal development, immune cell migration, and cancer metastasis. These findings suggest that migratory signals result in expression changes of post-translational modifications of splicing or polyadenylation factors, leading to splicing events that generate promigratory isoforms. We summarize this recent progress and suggest emerging technologies that may facilitate a deeper understanding of the role of alternative splicing and polyadenylation in cell migration.